eso9625 — Photo Release

Supernova 1996X in NGC 5061

18 April 1996

On April 12, two observers in Australia and Japan independently reported the sudden appearance of a supernova in the southern elliptical galaxy NGC 5061, a prominent object in the Centaurus constellation (IAUC 6380). The magnitude was about 13 (visual) and the exact position was measured as R.A. = 13h 18m 01.13s, Decl. = -26d 50m 45.3s (equinox 2000.0), or about 52 arcsec west and 31 arcsec south of the galaxy's center.

The 'new' star is seen on a composite. The left photo is a 10 second exposure with the EFOSC1 multi-mode instrument at the ESO 3.6-metre telescope (the supernova is indicated with an arrow. The right photo is a reproduction from the Digital Sky Survey (DSS), based on plates obtained with UK Schmidt telescope (It is copyrighted by the Space Telescope Science Institute; STScI Digitized Sky Survey, 1993, 1994, AURA, Inc. all rights reserved). The two photos cover the same field; North is up and east is to the left.

In order to follow this discovery up, ESO astronomers Stefano Benetti and Ferdinando Patat obtained a spectrum (range 4650 - 6670 Angstrom (A), resolution 2.9 A) on Apr. 14 with the ESO 1.5-metre telescope and the Boller & Chivens spectrograph (On the graph, the abscissa unit is Angstrom; the ordinate is relative intensity). They were able to confirm that SN 1996X is a type-Ia supernova, about one week before maximum light (IAUC 6381). The spectrum is dominated by P-Cyg type-lines of intermediate-mass elements superimposed on a blue continuum. Visible lines include Si II at 5051 A, 5958 A, and (very strong) 6355 A; Si III at 5740 A; Fe III at 5129 A; and S II at 5322, 5468, 5612, and 5654 A. The expansion velocity deduced from the minimum of Si II at 6355 A, corrected by the redshift (2032 km/sec) of the parent galaxy (NGC 5061), is about 11 150 km/sec.